Mailspring doesn't have an option to turn off updates. We release fixes often (including security fixes), and supporting people on old versions is hard for a small project, so we'd rather you stayed current. If you need to hold a version anyway — for example because a newer release dropped support for your operating system — here's how updates work.

• Windows and macOS: Mailspring checks updates.getmailspring.com in the background, downloads new versions, and installs them the next time it restarts. To stop this, block updates.getmailspring.com in your firewall or hosts file. On macOS you can additionally make the app read-only: chmod -R a-w /Applications/Mailspring.app.
• Linux (.deb / .rpm): Mailspring never installs updates itself. When one is available it shows a notification with a Download Now button that opens the download page, and it's up to you to install the package. Nothing to disable.
• Snap: updates come from snapd. Use sudo snap refresh --hold mailspring to pause them, or install the .deb instead.

Getting an older version

Every release is listed under Releases on the Mailspring GitHub project with installers for each platform. Older versions of macOS and Windows are supported until Electron drops them; check the release notes for the minimum OS version.